1. Field of the Invention
The invention relates to a control system for a clutch, which is configured to bring mutually opposed engaging elements into contact with each other, so as to couple the engaging elements with each other such that torque can be transmitted therebetween.
2. Description of Related Art
A hybrid vehicle having an engine and a motor as driving power sources is described in Japanese Patent Application Publication No. 2010-143365 (JP 2010-143365 A). When the hybrid vehicle runs in an EV running mode, using only power of the motor as driving force, the engine is cut off from a power transmission system. More specifically, the engine and the motor are connected to each other via a clutch. The clutch is a known dry-type single-disc clutch, and includes a clutch disc that is disposed between a pressure plate and a flywheel such that it is movable in the axial direction. The clutch is also provided with a diaphragm spring that applies spring force to the pressure plate. Further, a hydraulic actuator is provided for pressing a radially inner portion of the diaphragm spring, so as to reduce a load with which the diaphragm spring presses the pressure plate. Accordingly, when the load with which the diaphragm spring is pressed by means of the hydraulic actuator is small, the clutch disc is sandwiched between the pressure plate and the flywheel so as to transmit torque therebetween. When the load with which the diaphragm spring is pressed by means of the hydraulic actuator is large, the pressure plate moves away from the clutch disc, so that torque is inhibited from being transmitted via the clutch.
The clutch constructed as described above is operable in a fully engaged state in which torque is completely transmitted between the engine and the motor, a fully released state in which torque is inhibited from being transmitted, and a slipping state in which torque is transmitted while the clutch disc is slipping. Accordingly, friction surfaces of the clutch disc and the pressure plate or the flywheel become inevitably worn. If the friction surfaces are worn, the position of the pressure plate when it starts being engaged with the clutch disc changes, or the position of the pressure plate in the fully engaged state changes, which may result in reduction of the clutch controllability. Therefore, a controller described in JP 2010-143365 A sets the amount of movement of the hydraulic actuator measured when the pressure plate is completely engaged with the clutch disc, as a reference point for use in control of engagement or release of the clutch, and corrects the reference point when the amount of movement changes, by an amount corresponding to the change.
In a clutch constructed like the clutch as described in JP 2010-143365 A, if a clearance between the pressure plate and the clutch disc in the fully released state is large, it may take a long time from the time when the pressure plate starts being moved, to the time when the pressure plate contacts with the clutch disc, and the control response may be reduced accordingly. On the other hand, the balance of the mass of the clutch disc is not uniform in the circumferential direction, or the radial direction, or the axial direction, and, as a result, the clutch disc may undergo plane deflection while it is rotating. Therefore, if the clearance between the pressure plate and the clutch disc in the fully released state is reduced so as to curb reduction of the control response as described above, unintended transmission of torque may take place, due to plane deflection of the clutch disc.